In a tire air or pneumatic pressure monitoring device such as that described in Japanese Patent Application Publication No. 2010-122023, a transmitter sends a wireless signal at a constant, prescribed rotational position. When the wireless signals are received on the side of the vehicle, the rotational positions of respective wheels are detected. The wheel position associated with a transmitter corresponding to the rotational position that is best in synchronization with the output period of the wireless signal among the rotational positions of the wheels is determined as the position of the wheel (wheel position) with which that transmitter is associated.
According to the device described above, the rotational position of a transmitter is determined from the output value of an acceleration sensor, and a wireless signal is output at the time when the output value of the acceleration sensor assumes the prescribed value. At this instance, when a braking control such as an ABS control is performed, since the wheel under control is subject to longitudinally vibrate with an extremely short period to disturb the detected value of the acceleration sensor, the transmitter is likely to determine the rotational position erroneously and output a wireless signal at a rotational position different from the prescribed rotational position. Consequently, because of the determination of the wheel position of the transmitter based on the erroneous data on the rotational position, the determination of the wheel position is likely to be delayed.